The stress-inducible p53 protein acts as a central signal transduction node in the apoptotic response to DNA damage, mainly through its ability to transactivate intrinsic (mitochondrial) and extrinsic (death-receptor) pathway genes (Vousden and Lu, 2002). However, ample evidence supports the existence of p53-independent apoptotic responses to DNA damage. Most convincingly, in Drosophila and mouse p53 null embryos, several cell types undergo apoptosis in response to irradiation (IR), but with slower kinetics than p53+/+ cells (Frenkel et al., 1999; Wichmann et al., 2006).
Candidate p53-independent apoptotic pathways have recently emerged from in vitro studies. ATM/ATR-activated ABL, Chk1 and Chk2, for instance, can upregulate p73 protein levels via diverse mechanisms in genotoxically challenged p53-deficient cells, restoring transactivation of PUMA and other proapoptotic p53 targets (Gong et al., 1999; Roos and Kaina, 2006; Urist et al., 2004; Yuan et al., 1999). p53-independent coupling of DNA damage to mitochondria can also occur through translocation of the nuclear orphan protein Nur77 into the cytosol, activation of nuclear and/or cytosolic caspase-2, or de novo ceramide synthesis by mitochondrial ceramide synthase, all converging on caspase-3 activation (Kolesnick and Fuks, 2003; Li et al., 2000; Lin et al., 2004; Zhivotovsky and Orrenius, 2005). Other p53-independent processes, involving MAPKs (e.g., SAPK/JNKs, p38) and the transcription factors E2F1, NF-κB and FOXO1, couple DNA damage to caspase-3 activation independently of mitochondria by upregulating death-receptor pathway genes including CASP8, whose product cleaves caspase-3 (Afshar et al., 2006; Huang et al., 2006; Kasibhatla et al., 1998; Yount et al., 2001). Whether any of the p53-independent apoptotic pathways also operate in vivo remains an active field of investigation.
Radioresistant/chemoresistant p53 mutant human cancer cell lines cultured in vitro can be induced to die after genotoxic stress upon pharmacologic or RNAi targeting of DNA damage-response (DDR) kinases involved in intra-S and/or G2/M checkpoint control, including ATM, ATR, Chk1, Chk2, Polo-like kinases (Plks), and as most recently shown, the p38/MAPK-activated kinase MAPKAPK2 (MK-2) (Bunz et al., 1998; Castedo et al., 2004a; Chan et al., 2000; Chen et al., 2003; Collis et al., 2003; Reinhardt et al., 2007; Roninson et al., 2001; Zhou and Bartek, 2004). Interestingly, such treatments might spare cells endowed with wild-type p53, presumably because their intact G1 checkpoint enables them to repair and thus survive DNA damage (Chen et al., 2006; Mukhopadhyay et al., 2005; Reinhardt et al., 2007). Although the observed sensitization of, and selectivity for, p53 mutant cells is at the root of anticancer strategies that target DDR kinases, none of these concepts have been rigorously tested in vivo in an animal model (Garber, 2005; Kastan and Bartek, 2004; Kawabe, 2004; Zhou and Elledge, 2000). Furthermore, the p53-independent cell death program triggered by DDR kinase inactivation remains elusive, with contradictory results as to the involvement of certain caspases and Bcl-2 family members in the regulation of apoptotic or reproductive cell death (i.e., ‘mitotic catastrophe’; see, for example, Castedo et al., 2004a).